New laser systems for laser materials processing

In the last decades, diode laser systems conquered the spectral range step-by-step from conventional gas lasers, wherever they can match or outperform in optical specifications. Although highly anticipated in the ultraviolet wavelength range, for instance in high-resolution lithography, biological and medical fluorescence applications or holography, cw single frequency operation of sufficient power has been a challenge for diode or other solid state laser systems. Currently this scope is still dominated by the HeCd gas laser, emitting at 325 nm with powers of up to 100 mW. In this paper we present a diode laser system emitting at 325 nm offering the same output power by efficient second harmonic generation (SHG) of a master oscillator power amplifier (MOPA) at 650 nm. For the master oscillator a ridge waveguide diode is anti-reflection coated and used in an external cavity diode laser (ECDL) with grating feedback in Littrow configuration. This setup features a MHz line width (coherence length of 100m), a coarse tuning range from 649 nm to 657 nm and a mode hope free tuning of 20 GHz. In a second step, we use a tapered amplifier to boost the output from the ECDL to a level of 400 mW, for powering an efficient second harmonic generation process in an enhancement cavity. Faraday isolators on both ends of the amplifier stage prevent back reflection and stabilize the single mode operation of the system. Together with astigmatism compensation this yields to a high spatial quality (M²<1.5) of the amplified beam. The frequency doubling is achieved by using a four mirror bow-tie enhancement resonator fitted with a Beta-Barium Borate (BBO) crystal. The cavity length is actively locked to the laser frequency using the Pound-Drever-Hall method. With this set-up, stable and reliable laser operation is achieved. After a few minutes warm-up time, fixed frequency and tunable UV output power of more than 100 mW could be generated, opening this important wavelength range for future product development.

In this work we report on high-power diode laser modules with enhanced spectral brightness by means of volume holographic gratings for wavelength stabilization. High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser modules is an important means for more efficient pumping of solid-state lasers with a narrow absorption bandwidth. However, for efficient and reliable wavelength stabilization the parameters of the volume holographic grating and the parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the volume holographic grating, the reflectivity of the diode laser bar and the angular and spectral emission characteristics of the diode laser bar. In addition, the lateral structure of the diode laser bar and the microoptical elements for beam shaping have to be considered. In this paper we present a detailed characterization of different diode laser systems with wavelength stabilization in the spectral range from 790 - 1000 nm. The laser modules are divided into systems with and without fiber coupling. We will present data for a wavelength stabilized single diode laser bar with an output power of 69 W at a wavelength of 808 nm. Another example is a wavelength stabilized fiber-coupled diode laser module with an output power of 456 W for a fiber with a core diameter of 400 &mu;m (NA 0.22).

In the last few years, fiber-coupled diode lasers have shown massive applications in many fields of communication and scientific research. Particularly, the pumping of solid-state lasers is a key application for more powerful diode lasers enabling good solutions in various laser micro methods like metal cutting, sintering, structuring as well as drilling. In this work, a simple beam shaping method is demonstrated for coupling a high-power semiconductor laser diode into multi-mode fiber optic using optical lenses. The optical lenses as beam transformation components are utilized to reshape the asymmetrical irradiation of the diode laser bar and to circularize the laser beam. Using this simple method, compact, high-output-power, and high-brightness fiber-coupled laser diode systems are achieved. The obtained results show that the entire optical coupling efficiency of the system is more than 65%.

New wavelengths, high brightness and increasing reliability are breaking new ground for diode lasers. The higher brightness allows direct fiber coupling of several 100 W in 200 μm and 400 μm fibers and therefore i.e. the pumping of fiber lasers. Such high power fiber coupled laser diodes combined with fast beam deflection units (galvo scanners) have been implemented in industrial processes and used for quasi‐simultaneous welding of polymers, heat treatment in the sub millimeter range or selective soldering in solar cell production. Besides pumping of solid state lasers, high power diode lasers become more and more a competitive tool for many applications in material processing.

In this paper we discuss how different feedback gratings affect the tuning range and the output power of external feedback diode laser systems. A tunable high-power narrow-spectrum external-cavity diode laser system around 455 nm is investigated. The laser system is based on a high-power GaN diode laser in a Littrow external-cavity. Both a holographic diffraction grating and a ruled diffraction grating are used as feedback elements in the external cavity. The output power, spectral bandwidth, and tunable range of the external cavity diode laser system are measured and compared with the two gratings at different injected currents. When the holographic grating is used, the laser system can be tuned over a range of 1.4 nm with an output power around 530 mW. When the ruled grating is used, the laser system can be tuned over a range of 6.0 nm with an output power around 80 mW. The results can be used as a guide for selecting gratings for external-cavity diode lasers for different requirements.

High Power Laser Diodes (HPLD) are increasingly used in different fields of applications such as Scientific, Industrial, Medical and Defense. Our significant performances improvements (especially in peak power, divergence and efficiency) and a reproducible manufacturing process have led to reliable, highly robust components. For industrial applications these devices are used predominantly for pumping of solid state lasers. Due to the drastically falling price per watt they are more and more replacing flash lamps as pump sources. Their compact size and high reliability make diode laser arrays ideally suited to this demanding market. For Scientific applications, the brightness as well as the cost are the two main drivers. In order to respond to this demand, QLD has focused its development on very high peak power bars associated with very high E/O efficiency. The level of energy QLD is achieving now has also pushed us to develop a power supply driver able to operate such diodes at up to 1000A under 100V.

High wall-plug efficiency and a wide range of available wavelengths make laser diode arrays preferable for many high-power applications, including optical pumping of solid state lasers. Recently, we designed and fabricated InGaAsP/InP arrays operating at 1.5-&#956;m and In(Al)GaAsSb/GaSb arrays operating at 2.3-&#956;m. We have demonstrated a high continuous-wave (CW) output power of 25 W from a one dimensional laser array and a quasi-CW (q-CW) output power of 110 W from a two dimensional laser array both operating near 1.5-&#956;m. We have obtained a CW output power of 10 W from the 2.3-&#956;m laser array. The 1.5-&#956;m arrays are suitable for resonant pumping of erbium doped solid-state lasers, which require high power optical sources emitting in the narrow erbium absorption bands. Long current-injection pulses produce a considerable temperature increase within the diode laser structure which induces a red-shift of the output wavelength. This thermal drift of the laser array emission spectrum can lead to misalignment with the erbium absorption bands, which decreases pumping efficiency. We have developed an experimental technique to measure the time dependence of the laser emission spectrum during a single current pulse. From the red-shift of the laser emission, we determine the temperature of the laser active region as a function of time. The spacing between the individual laser emitters has an effect on the array heating. In steady state operation, this spacing is a contributing factor in the non-uniformity of the thermal field within the bar, and thus to the overall thermal resistance of the laser bar. Under pulse operation, the transient heating process can be divided into three time periods; each with its own heat transport condition. It was shown that in the initial period of time the heat propagates within the laser bar structure and the laser bar design (fill factor) strongly affects the active region temperature rise. In the later periods the temperature kinetics is insensitive to the fill factor. This analysis has been verified in experimental studies using the 1.5-&#956;m laser arrays.

4.03] Clinical application of 1470nm diode laser system for treatment of hyperplastic inferior nasal turbinate compared to 940nm diode laser system

There is an increasing demand for high-power, high-brightness diode lasers from 8xx nm to 9xx nm for applications such as fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. The kilowatt CW fiber laser pumping (915 nm - 976 nm), in particular, requires the diode lasers to have both high power and high brightness in order to achieve high-performance and reduced manufacturing costs. This paper presents continued progress in the development of high brightness fiber-coupled product platform, <i>element^TM</i>. Further brightness improvement and power-scaling have been enabled by both the rise in chip brightness as well as the increase in number of chips used to couple into a given numerical aperture. We have developed a new generation of high power broad area laser known as reduced-mode diode (REM-diode) which suppresses many of the higher order modes in the slow axis and reduces divergence up to two times at the same operating conditions. To date, we have achieved slow-axis brightness as high as 4.3 W/mm-mrad for devices with thermal resistance of ~2.5 C/W. As a result, we have achieved &gt;75 watts from a 1&times;6 <i>element^TM</i>in the 9xx nm spectral range; and 177 watts of peak power from a 2&times;6 <i>element^TM</i>. We have also improved our optics for fiber-coupling which accommodates 7 emitters per polarization in the same numerical aperture. Using this configuration, we project 200 watts of peak power from a 2&times;7 <i>element^TM</i> with a reliable product at 176 W of power from 105 &mu;m and 0.15 NA fiber. REM-diodes can also be wavelength stabilized using VBGs. The reliability of REM-diodes are equal or better than broad area lasers (BALs). We present current status on ongoing reliability assessment of chip-on-submount.

Gradient index micro lenses for laser diode applicationsR. Goering, T. Possner, B. MesserschmidtFraunhofer Institution for Applied Optics and Precision EngineeringSchillerstrasse 1 , 07745 Jena, GermanyABSTRACTCollimating and diverging cylindrical micro lenses with N.A. up to 0.60 fabricated by thermal silver ion exchangein a special designed glass type are reported. Cylindrical microlens arrays are fabricated by field assistet ion-exchange. Applications to fiber coupling as well as to diode bar collimation are discussed.Keywords: ion-exchange, GRIN-components, laser diode beam shaping, laser bar collimation, fiber coupling1. INTRODUCTIONMicrooptical GRIN-lenses with high N.A. are very useful for the transformation of the strongly divergent andasymmetric beam emitted from semiconductor laser diodes and laser diode bars to a well-collimated beam orto a focused spot. Preferred applications are the coupling between laser diodes and optical fibres, the pumpingof solid state lasers as well as optical scanning. High N.A., a nearly diffraction limited pertormance and lensdimensions of a few hundred microns are required. In a previous paper we presented the formation of one-dimensional cylindrical microlenses by a two step silver ion-exchange process1. Here we would like to discussthe optimization of these components and a number of new elements fabricated by similar ion-exchangeprocesses and applications which could not be realized by standard SELFOC-lenses.Figures 1 and 2 show flat GRIN-components and ion-exchange techniques for their realization. The formationof diverging lenses is possible in a single ion-exchange step. For the generation of cylindrical collimating lenseswe use a two step process, proposed by Ohmi2. Cylindrical microlens-arrays are produced by a field-assistedexchange using a masked substrate.Fig. 1: Thermal ion-exchanged GRIN components

The National German research project "MDS", which stands for the German equivalent of "Modular Diode Laser Beam Tools", is now in its fifth year. The aim of the project is the development of new diode laser technology as well as novel applications, especially taylored to the special features of high power diode lasers. The project has a strong vertical structure: All necessary technologies for a successful performance of the project, i.e. chip-technology, laser-system-technology and applications are represented in the project. The aspect of laser systems can be divided into two fields: one is of course improving the beam quality and power, i.e. brilliance of conventional diode laser systems; this can be reached by further development of the semiconductors the other is using the individual bars and assembling them in a special way, which is adapted to the application. That's what is meant by the word "modular". This paper will mainly focus on the special systems and their applications and illuminate the actual status as well as the hurdles, which had to be overcome during the project.

Q-switched Nd-based lasers are used for a variety of medical and manufacturing applications and benefit from a high efficiency and stable diode pump, but limited commercially available solutions exist for single spatial mode direct pumping of Nd-based lasers at 885 nm. To satisfy this need, Freedom Photonics has developed ultra-robust watt-class diffractionlimited 885 nm diode lasers. We have demonstrated single-mode 885 nm diode lasers with an output power of <1.8 W (1 W rated) and 49% electricalto-optical efficiency. We employ facet passivation methods to protect against catastrophic optical mirror damage (COMD), and the optical power density we have demonstrated points to an effective facet passivation strategy. These devices may be used as high efficiency pumps for Nd-doped solid-state lasers.

Q-switched Nd-based lasers are used for a variety of medical and manufacturing applications and require extremely high efficiency and stability from the diode pump, but limited commercially available solutions exist for single spatial mode direct pumping of Nd-based lasers at 885 nm. To satisfy this need, Freedom Photonics has developed ultra-robust watt-class diffraction-limited 885 nm diode lasers. We are in the process of performing full reliability and lifetime assessments on our watt-class single-mode 885 nm diode lasers. To achieve high reliability, we employ facet passivation methods to protect against catastrophic optical mirror damage (COMD). Additionally, progress on the development of VBG-locked packages will be discussed. These devices may be used as reliable, high efficiency pumps for Nd-doped solid-state lasers.

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A major problem using laser diodes for longitudinal pumping of solid state lasers is the poor beam quality and the non symmetric beam profile of the diodes as the astigmatic beam emitted from the array of diodes exhibits a limited focusability in the direction of the slow axis of the diodes. To overcome this problem an optic which turns the astigmatic beam of a stacked diode array into a radially symmetric (stigmatic) beam has been designed. This symmetric beam is then focused into an axially water cooled disk laser to serve as a longitudinal pump source. Up to now different laser crystals have been investigated as laser source. Using Yb:YAG an output power of 60 W in qcw operation has been realized whereas Nd:YVO<SUB>4</SUB> delivers a qcw power of 113 W. The optical to optical efficiencies are 18% and 32.3% respectively.